My (simple) understanding is this: You can use a pistol frame/receiver to make a rifle (not sbr). But, you cannot take a rifle receiver and convert it into a pistol. I had a Mac 10 Carbine - the lower frame/receiver was manufactured as a carbine, not a pistol (thus serial # had a "c" in it). You COULD NOT put a pistol upper on it. Now, Mac 10 pistols have carbine kits that you can mount that are completely legal.